The present invention relates to a control for a fluid pump (an oil pump or a water pump) of an engine.
In general, a lubrication apparatus and a cooling apparatus are supplied with an engine. The lubrication apparatus is an apparatus for reducing the frictional resistance by using oil for each section of the engine. The lubrication apparatus uses an oil pump coupled to a crankshaft of the engine to feed oil to a lubrication passage. The cooling apparatus is an apparatus for keeping the temperature at which the engine can continue rotating with stability. The cooling apparatus feeds cooling water to a passage disposed in a cylinder block and a cylinder head of the engine to prevent the engine from overheating. The cooling apparatus uses a water pump coupled to the crankshaft of the engine to circulate the cooling water. These apparatuses are required for maintaining the engine in the normal operating condition. These apparatuses have an effect of improving the engine efficiency, that is, the fuel efficiency.
There is a problem with a method for driving such fluid pumps (oil pump and water pump) in the lubrication apparatus and the cooling apparatus. Since these pumps are connected to the crankshaft of the engine, a driving force is provided to these pumps in accordance with the engine rotation. The rotational speed of each pump is determined in accordance with the engine rotational speed. The discharge capacity of the pump increases as the rotational speed of the engine increases. If the engine rotational speed is low, the discharge capacity of the pump is small. Such a small discharge capacity may reduce the fuel efficiency, especially in the engine having a variable valve driving mechanism and/or a variable compression ratio mechanism because responsiveness of those mechanisms may deteriorate when the engine rotational speed is low. If the pump is configured to generate a sufficient pump output when the engine rotational speed is low, work by the pump is excessive under the condition where the engine rotational speed is high and the engine load is low. Such redundant work by the pump reduces the overall engine efficiency.
As shown in FIG. 11, it is preferable that ideal oil pressure characteristics of an oil pump (shown by solid lines) generate a sufficient high oil pressure to meet a requirement of the hydraulic system when the engine rotational speed is low, and generate a sufficient low oil pressure appropriate to the condition where the engine rotational speed is high and the engine load is low, as compared with the characteristics of a conventional pump (shown by dashed line). Thus, there is a need for a fluid pump that is capable of producing a desired output independently of the engine rotational speed.
As one of the solutions to meet such need, there is an electrically-driven pump. The electrically-driven pump can control the rotation of the pump independently of the engine rotation because it utilizes the driving force of a motor for rotating the pump. There are two types of the electrically-driven pump. One is a brush motor and the other is a brushless motor.
Although the electrically-driven pump can control a flow rate of the pump independently of the engine rotational speed, there are some problems. If a brush motor is used, its reliability is low because it is susceptible to aging and failure due to wear of the brush. If a brushless pump is used, a PDU (power distribution unit) is required for controlling the magnetic field by the three-phase lines, which increases the weight of the pump system and hence decreases the fuel efficiency.
Thus, there is a need for a pump that meets ideal oil pressure characteristics as shown in FIG. 11 (in case of an oil pump) and other ideal characteristics regarding the rotational speed and the water temperature (in case of a water pump) while implementing high reliability and high fuel efficiency.
On the other hand, a sliding mode control is known in the field of engine control. The sliding mode control is capable of adjusting the characteristics that a controlled variable follows and converges to a desired value (refer to the Japanese Patent Application Unexamined Publication No. 2003-155938). The sliding mode control can reduce redundant work and improve the fuel efficiency
Further, a control using a delta-sigma (ΔΣ) modulation algorithm is known. Such a delta-sigma modulation algorithm can implement a high accurate control regardless of variations in the operating characteristics of a controlled object as long as the controlled object has a capability of generating an appropriate output in response to an on/off control input (refer to the Japanese Patent Application Unexamined Publication No. 2003-195908).